There is great interest in the generation of images using thermal transfer technology, particularly in the generation of images using near-infrared laser devices. In thermal transfer imaging, an image is formed by transfer of a colorant (e.g., a dye or pigment) from a donor layer of a donor element to an image-receiving layer of an image-receiving element under the influence of energy from a thermal resistive printhead or a laser. The donor element, which is generally a sheet having a coating layer containing a transferable colorant, and an image-receiving layer of an image-receiving element are brought either into close proximity or into contact with each other to form a donor-receiver assembly. Energy is applied to a side of the donor element opposite the donor layer to transfer an image from the donor to the receiver. When the energy source is a laser, an infrared absorber can be present in one or both of the donor element and image-receiving element. Most commonly, the infrared absorber is present only in the donor element. When the donor-receiver assembly is patternwise exposed to infrared radiation, for example from a scanning infrared laser source, the radiation can be absorbed by the infrared absorber and converted to heat, which causes transfer of colorant from the donor layer of the donor element to the image-receiving layer of the image-receiving element in the imaged areas. The resulting image can be monochrome, or by repeating the process with donors of various colors, a multi-colored image can be produced on a common image-receiving element. Such a process is useful for generating color proofs, for instance.
Thermal transfer imaging may be utilized in the production of color proofs. Pre-press or off-press color proofing is used by printers to simulate the images that will be produced by a printing process. Pre-press color proofing systems include overlay proofing systems and integral proofing systems.
In an overlay proof, each printing color is generally segregated onto a separate transparent sheet or film, known as an overlay. The individual overlays are assembled in registration to make the overlay proof, which is viewed as a composite against an appropriate background (e.g., an opaque reflective white sheet), to predict the appearance of a printed image.
In an integral proof, all printing colors are generally transferred to one medium. One commonly used method of obtaining an integral proof is by a “surprint” technique. In a surprint technique, the transfer process described above is repeated using different donor elements, generally representing different colors, and the same image-receiving element. In this manner, several monochrome images are superimposed in register on a common receiver element, thereby generating a multi-color image in a single-sheet format. A proof made by the surprint technique is also known as an “overprint” proof, and the two terms are used interchangeably herein.
Both overlay proofs and surprint proofs are commonly used as “contract proofs.” A contract proof serves as a promise by the printer to a customer that a proofed image will be duplicated by the printing process when press prints are made. Therefore, the printer desires to have proofs that can most accurately predict the image that will be reproduced on press. It is desired that the color proof will accurately represent the image quality, details, color tone scale and, in many cases, the halftone pattern of the prints obtained on the printing press. A proof is also useful for checking the accuracy of the color separation data from which the final printing plates or cylinders are made.
The surprint process is ideally suited for processing images by a thermal transfer imaging procedure using digitally stored information. The surprint process has the additional benefit of not requiring chemical processing and of not employing materials that are sensitive to ambient white light. The process is particularly suited to the color proofing industry, where color separation information is routinely generated and stored electronically, and the ability to convert such data into hardcopy via digital address of “dry” media is seen as an advantage.
In thermal transfer imaging, the transfer of colorant can occur via mass transfer or dye transfer. In a mass transfer system, the majority of the material on the donor element, for example, the colorant, binder, and any additives, is transferred to the image-receiving element. For a laser thermal transfer system, this occurs by ablation, where gases are generated by the laser in the donor to explosively propel the donor material to the receiver. In a dye transfer system, only the colorant is transferred from the donor element to the image-receiving element. That is, the colorant is transferred unaccompanied by the binder or other additives. Colorant transfer under influence of a laser is by dye sublimation.
Thermal transfer imaging systems produce excellent results with respect to approximating images that can be produced on-press using standard colors, such as the process colors cyan, magenta, yellow, and black, or even custom (e.g., PANTONE) colors. In addition to standard or custom colors, however, a press print will sometimes require the use of an image or finish that cannot be duplicated by merely adjusting light absorption properties of the printed image. For example, specialty finishes such as metallics, fluorescents, and pearlescents can be used on-press to create interesting visual effects. Such visual effects are often realized by the use of specialized pigments or coatings, and are not always easy to reproduce using a digital proof created from a thermal transfer imaging system.
One technique that is used on-press to create visual interest is the application of a spot varnish or gloss to selected portions of a printed image to enhance the glossiness of the selected portions. If the varnish is applied while the press ink is still wet, the technique is called “wet trap varnish.” If the ink is allowed to dry first, the technique is called “dry trap varnish.” Spot varnishing is often used in the production of high-quality prints.
However, methods for mimicking the on-press application of a spot varnish in a pre-press digital proof have not always proven to be straightforward. U.S. Pat. No. 6,060,210 to Eda, et al. reports a varnishing film and a method of adjusting the surface gloss of a pre-press color proof using the varnishing film. The varnishing film comprises an adhesive transparent resin layer containing transparent matting agent and a transparent photosensitive layer provided on a transparent support. The method requires exposure and development of the photosensitive layer to form a special film resulting in a gloss finish on the surface of a proof. The photosensitive layer requires liquid development to form a gloss version of the image, complicating the proofing process.
Another method of representing a spot varnish is to produce a full-color surprint proof, and then use a transparent overlay that is colored only in the areas to which a spot varnish is to be applied, to represent the spot varnish.
Presently, it is not uncommon for a pre-press surprint proof to completely lack a surface finish that can predict the result that will be obtained on-press when a spot varnish is employed. There remains a need for a process by which an accurate proof that can emulate the appearance of a spot varnish applied on-press can be made in a simple fashion.